1. Field of the Invention
The present invention relates to a control device for a cylinder injection type internal-combustion engine which directly supplied fuel to a cylinder of an internal-combustion engine and, more particularly, to a control device for restraining the deterioration of combustion.
2. Description of Related Art
A cylinder injection type internal-combustion engine designed to directly supply fuel to the cylinders of an internal-combustion engine has been offering the hope of achieving a reduced displacement of exhaust gases, lower fuel cost, higher output, and improved drivability.
In a cylinder injection type internal-combustion engine, fuel is injected around a spark plug according to an ignition timing immediately before ignition, thereby producing flammable fuel around the spark plug to perform stratified charge combustion. At this time, the fuel-air mixture is unevenly distributed in a cylinder; the mixture has a value in the vicinity of a stoichiometric air-fuel ratio around the spark plug, whereas it is lean in other area. As a result, the apparent air-fuel ratio of the amount of inlet air to the amount of fuel supplied indicates a markedly lean fuel-air mixture.
Hence, extremely lean operation with an apparent air-fuel ratio of 30 or more can be accomplished, enabling a reduced displacement of exhaust gas and lower fuel cost to be achieved.
Further, since there is less fuel-air mixture around the spark plug, there is accordingly less end gas which is responsible for knocking, so that the compression ratio of an engine can be increased. In addition, the fuel which vaporizes in a cylinder takes the heat of vaporization; therefore, the density of introduced air rises with a resultant increase in volumetric efficiency, thus permitting higher output.
Since fuel is directly injected into a cylinder, it is unnecessary to consider the adherence of the fuel to the wall of an inlet pipe or a suction valve, allowing the mixture to be further lean. In addition, there is less time lag from the injection of fuel to the generation of output, enabling improved drivability.
As this type of cylinder injection internal-combustion engine, the one disclosed, for example, in Japanese Unexamined Patent Publication No. 4-187819 has been known.
In the cylinder injection type internal-combustion engine, although the apparent supply air-fuel ratio is 30 or higher and it indicates an extremely lean mixture, the air-fuel ratio in the area where actual combustion is taking place is in the vicinity of the stoichiometric air-fuel ratio, namely, 14.7. This means that the engine is operated in the vicinity of an air-fuel ratio of 16 at which a large amount of nitrogen oxide (NOx) is generated, whereas a lean burn engine is operated at an air-fuel ratio of approximately 20; therefore, a large amount of exhaust gas is recirculated to the intake end to reduce the generated nitrogen oxide.
Such a cylinder injection type internal-combustion engine performs the stratified charge combustion according to delicate timings for fuel injection and ignition. For this reason, if combustion deteriorates, it has been difficult to restrain the deterioration.
It has also been difficult to restrain the deterioration of combustion while restraining damage to the characteristics of the aforesaid cylinder injection type internal-combustion engine at the same time.
It has also been difficult to achieve a highly reliable device for securely restraining the deterioration of combustion.
It has been preferable to control the deterioration of combustion for each cylinder in order to control damage to the characteristics of the cylinder injection type internal-combustion engine.